The present invention is directed to a power supply which maintains a substantially constant output voltage when connected either to a low or high level of voltage input. This type of power supply has particular utility with electronic equipment, such as computers, which are designed for use in more than one environment where different levels of A.C. 60 cycle power are provided. The most common levels encountered are 110 volts and 220 volts A.C. The equipment serviced by the power supply requires that the output of the power supply, which is generally a D.C. voltage, be maintained at a substantially constant value irrespective of the level of the A.C. input voltage. Generally speaking, in order to meet this requirement, a method is employed in which the magnitude of the A.C. input voltage is recognized and the internal circuitry of the power supply is adjusted so as to maintain the output voltage substantially constant. The most commonly used method of achieving this adjustment is to switch the power supply from a full wave rectifier operation to a voltage doubler operation. Various techniques have been used to accomplish the switching, e.g. from manually switching after identification of the level of the A.C. input voltage to systems which automatically detect the level of the A.C. input voltage source when the power supply is connected to the source. The automatic systems use electronic switching means to switch between the full wave rectifier operation and the voltage doubler operation in accordance with the level of the A.C. input voltage.
In U.S. Pat. No. 3,713,018 to C. T. Braunschweig, there is disclosed a capacitor charging circuit which, in one embodiment, uses a hand operated switch that is opened or closed as a function of the level of the A.C. voltage to be applied to the charging circuit to switch between the full wave rectifier operation and the voltage doubler operation. In another embodiment of the same patent, an electronic switch (triac), responsive to a voltage magnitude detecting circuit, is used to automatically provide the switching action.
In U.S. Pat. No. 3,900,787 to M. Koster, there is disclosed an automatic system which uses a thyristor switch, activated by an input voltage level sensing circuit, for connecting the power supply in either the voltage doubler or the rectifier configuration.
In U.S. Pat. No. 4,590,546 by D. W. Maile, a triac is used to automatically switch between the voltage doubler and the full wave rectifier configurations. The triac is activated by the level of the A.C. voltage applied to the circuit's input. Also, In U.S. Pat. No. 4,608,500 to J. Togawa and U.S. Pat. No. 4,654,538 to P. R. Lethellier, a triac is used to switch between the voltage doubler and the full wave rectifier configurations.
All of these prior art systems operate with the possibility that the system will not be in the correct operating mode when the A.C. input voltage is applied to the power supply. For example, if the switch is manually operated and the power supply is placed in the low level A.C. operating mode but is, in fact, attached to a high level A.C. input voltage, the D.C. output voltage from the power supply will be excessive to the point that damage will occur to the serviced equipment. In those prior art systems which have automatic A.C. level detection, interruptions, defaults and/or spikes in the A.C. input voltage may cause the power supply to respond with an erroneous level detection, which in turn will place the power supply in the wrong operating mode for the A.C. input voltage.
If a delay is provided to eliminate the possibility of the power supply responding to discontinuities and/or spikes, it will also be slow in responding to the actual A.C. input voltage level which in turn may cause damage to the serviced equipment. The highest probability of equipment damage is related to the erroneous detecting of the A.C. input voltage as being of a low level when, in fact, the voltage is at a high level. As previously mentioned, with such an error, the output D.C. voltage will be exceedingly high and it is this high D.C. voltage, at the output of the power supply, which causes the equipment damage. Another error in detecting occurs when a high voltage level is indicated to be at the input when, in fact, the voltage level is low. This will result in a below normal D.C. level at the output of the power supply, which would, of course, effect equipment performance, but would not necessarily cause equipment damage.